1. Technical Field
The present invention relates to electromagnetic shields and, more particularly, relates to edge treatments for high efficiency electromagnetic shields.
2. Description of the Related Art
Metal cage shields on radio circuit boards are used to shield components from one another and also reduce electromagnetic emissions. Metal cage shields typically use straight edges at the point of contact with a printed circuit board. Sometimes, for solder reliability reasons, these shields have their edges bent outward.
Shields on radio circuit boards usually require apertures or openings in the shield because the signal must get out. These apertures in shields noticeably reduce the effectiveness of the shielding.
The inventors of the present invention have discovered that these practices compromise shielding effectiveness. A more effective shield having an improved edge treatment is provided by the invention as will be described below.
A conductive shield surrounds one or more radio components to shield them but some leakage of electromagnetic fields occurs. It has been discovered that these fields are formed primarily by eddy currents induced in the metal of the shield and that the emission through the air is strongest at the edges of the shield. It is a feature of the present invention that the edges of the surface metal of the cage shield are bent inwardly towards an interior of a cavity formed by the cage shield to eliminate this leakage.
Apertures or openings in the shield are required in the shield to allow signals to get in and out to and from the radio components. Most of the leakage of the electromagnetic fields can occur at these apertures. According to a further feature of the present invention, the edges of the shield at the apertures are bent inwardly towards a cavity formed by the shield for improved shielding and to eliminate more leakage of the electromagnetic fields. The proposed shield has inwardly folded edges to reposition the strong edge currents and confine their radiation within the shielded volume. Typically, this confined radiation is subsequently attenuated by losses in the interior of the shield, the packaging material of the radio components and by losses in the printed circuit board.
Metalization patterns on a printed circuit board provide a ground plane for shielding the portion below the conductive cage. Metalized through holes or vias connect the conductive cage to the ground place. When an aperture is created along an edge formed of the printed circuit board, the metalized through holes can be pointed inwardly to confine the radiation from the edges to within the shielded volume.